In certain types of electronic systems designed for the transmission and reception of acoustic or electromagnetic signals, the algorithms of the intelligence carried in the received signal are of such a nature that, in order for receivers to detect the intelligence with integrity, the receiver's automatic gain control (AGC) circuit must have a substantially long time constant, i.e. five or more minutes may be required. Sonar is an example of technology having the described signal requirements. However, it is also desirable to have a much shorter time constant if the amplitude of the gain-controlled signal significantly departs from the nominal desired amplitude level, such as, for example, by more than 3 dB. Previously, very long AGC time constants have not been sucessfully achieved by conventional means.
It is, therefore, an object of the present invention to provide new and improved signal processing systems, including AGC circuits, having very long time constants.
It is another object of the present invention to provide such systems wherein the time constant can be rapidly switched between one which is very long and one which is much shorter.